The using declaration and class members (C++ only)

A using declaration in a definition of a class A allows you to introduce a name of a data member or member function from a base class of A into the scope of A.

You would need a using declaration in a class definition if you want to create a set of overload a member functions from base and derived classes, or you want to change the access of a class member.

using declaration syntax

Read syntax diagramSkip visual syntax diagramusing typename::nested_name_specifierunqualified_id;::unqualified_id;
A using declaration in a class A may name one of the following:
  • A member of a base class of A
  • A member of an anonymous union that is a member of a base class of A
  • An enumerator for an enumeration type that is a member of a base class of A
The following example demonstrates this: 
struct Z {
  int g();
};

struct A {
  void f();
  enum E { e };
  union { int u; };
};

struct B : A {
  using A::f;
  using A::e;
  using A::u;
//  using Z::g;
};
The compiler would not allow the using declaration using Z::g because Z is not a base class of A.
A using declaration cannot name a template. For example, the compiler will not allow the following: 
struct A {
  template<class T> void f(T);
};

struct B : A {
  using A::f<int>;
};
Every instance of the name mentioned in a using declaration must be accessible. The following example demonstrates this: 
struct A {
private:
  void f(int);
public:
  int f();
protected:
  void g();
};

struct B : A {
//  using A::f;
  using A::g;
};
The compiler would not allow the using declaration using A::f because void A::f(int) is not accessible from B even though int A::f() is accessible.
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